Ionic liquids as solvents in headspace gas chromatography

ABSTRACT

A method of using an ionic liquid as solvent in headspace gas chromatography.

The present invention provides a method of using an ionic liquid assolvents in headspace gas chromatography.

The purity of a compound may affect many physical and chemicalproperties of the compound, for example, the electrical conductivity,luminescence, capacity for polymerization, and stability. Evenimpurities present in very low amounts, for example, 10⁻² to 10⁻⁹, maydeleteriously affect the properties of a compound or the analysis of thecompound, and thus, prevent a compound from being used in its intendedfield. This is especially true in the pharmaceutical industry whereimpurities in a drug compound may decrease bioavailability of the drug,and prevent the Food and Drug Administration from either approving thedrug, or approving a process to prepare the drug.

Volatile constituents of samples have been determined by gaschromatography (GC) advantageously for the following reasons: (i) GCprovides in a single analysis information about many impurities, notjust a single one, particularly because it gives very sharp separations,which allows one to analyze for impurities that differ only slightly inproperties, such as isomers; (ii) sensitive detectors allow one todetect impurities at very low concentrations; (iii) headspace gaschromatography can be combined with spectroscopic instruments foridentification of separated compounds; and (iv) accumulation techniquescan be used independently to reduce further the minimum detectableconcentration.

Other methods of gas analysis exist, such as mass spectrometry, IRspectroscopy and UV spectroscopy, for the determination of compounds intheir vapor state in the presence of liquids and solids. However, thesemethods are often insufficiently sensitive, and when several materialsare present in the vapor phase, they give inextricably complex results,since only the sum of the components can be measured.

Headspace gas chromatography (HSGC) generally consists of a static ordynamic headspace gas sampling device, which may be manually operated orautomated, and a gas chromatograph. The headspace sampling devicevariants allow for selectively volatilizing the volatile components of atest sample. A representative fraction or the total amount of thevolatile components is carried into the chromatographic column mountedin the oven of a gas chromatograph. Several variants of technicalrealizations are commercially available or can be handcrafted. Staticheadspace sampling devices apply, for example, balanced pressureinjection, loop-injection or syringe injection. In a first step, dynamicheadspace sampling device usually, but not exclusively, preconcentrate arepresentative fraction or the total amount of the volatile componentsof the test sample in a trap. In the second step, a representativefraction or the total amount of the preconcentrated volatile componentsof the test sample is then carried from the trap into thechromatographic column mounted in the oven of a gas chromatograph. Aflow of carrier gas carries the volatile components through thechromatographic column where they are separated. The separatedcomponents enter a detector, which determines the concentration or massflow of the components in the carrier gas.

Ionic liquids have been used as solvents for a number of reactions, forexample, Friedel-Crafts reactions (Adams, C. J., et al., ChemistryCommunications, 1998, pgs. 2097-2098; isomerisation of fatty acidderivatives (WO 98/07679, and U.S. Pat. No. 6,255,504); dimerization,co-dimerization and oligomerization of olefins (U.S. Pat. Nos.5,550,306, and 5,104,840); Diels-Alder reactions (Earle, M. J., et al.,Green Chemistry, 1999, vol. 1, pgs. 23-25); and hydrogenation reactions(Fisher, T., et al. Tetrahedron Letters, 1999, vol. 40, pgs. 793-794).

Disadvantages associated with using conventional solvents in headspacegas chromatography are (i) high vapor pressure which causes broadsolvent peaks in the chromatogram; (ii) limited temperature range ofapplication; and (iii) carry over from consecutive injections in the gaschromatograph. Therefore, it would be desirable to have a method thatallows for the detection, and/or quantification, and/or identificationof volatile components in a test sample without the disadvantagesarising from conventional solvents.

The invention provides the use of an ionic liquid as solvent inheadspace gas chromatography.

In another aspect the invention provides the use of an ionic liquid assolvent in headspace gas chromatography wherein said method comprisesdissolving or dispersing a sample in at least one ionic liquid andvolatilizing the volatile components of the sample.

In a further aspect the invention provides a method to detect volatilecomponents in a sample by headspace gas chromatography, wherein saidmethod comprises dissolving or dispersing a sample in at least one ionicliquid and volatilizing the volatile components of the sample. Forexample, the ionic liquid has essentially no vapor pressure.

According to another aspect the invention provides a method to quantifyand/or identify volatile components in a sample by headspace gaschromatography, wherein said method comprises dissolving or dispersing asample in at least one ionic liquid and volatilizing the volatilecomponents of the sample.

In another aspect the invention provides the use of an ionic liquid assolvent in headspace gas chromatography for the detection, e.g.quantification and/or identification, of impurities in a sample, e.g. ina pharmaceutical compound.

Hereinafter follows a brief description of the drawings:

FIG. 1 is a headspace gas chromatogram showing residual solvents in1-ethyl-3-methyl-imidazolium trifluoromethanesulfonate (I),1-ethyl-3-methyl-imidazolium bis-(trifluoromethanesulfonyl)-imidate(II), N,N-dimethylacetamide (III) and dimethylsulfoxide (IV).

FIG. 2 is a headspace gas chromatogram showing a tetrahydrofuran, THF,containing drug substance dissolved in 1-butyl-3-methyl-imidazoliummethane sulfonate (I) and in dimethylsulfoxide (II).

FIG. 3 is a headspace gas chromatogram showing dimethyl formamide (DMF),dimethylsulfoxide (DMSO), dimethylacetamide (DMA) anddiethylenegycoldimethylether (DIGLYME) dissolved in1-ethyl-3-methyl-imidazolium bis-(trifluoromehtanesulfonyl)-imidate (II)and 1-ethyl-3-methyl-imidazolium bis-(trifluoromehtanesulfonyl)-imidate(I).

FIG. 4 is a headspace gas chromatogram showing drug substance dissolvedin 1-ethyl-3-methyl-imidazolium bis-(trifluoromethylsulfonyl)-imidate(I) and in dimethylacetamide (DMA) (II).

The method of the invention uses ionic liquids as solvents in headspacegas chromatography to detect volatile components in a sample, whereinsaid method comprises dissolving or dispersing a sample in at least oneionic liquid and volatilizing the volatile components of the sample. Inanother embodiment of the invention, the method of the invention is usedto quantify and/or identify volatile components in a sample by headspacegas chromatography, wherein said method comprises dissolving ordispersing a sample in at least one ionic liquid and volatilizing thevolatile components of the sample. The volatile components of a samplecan be analyzed by the method of the invention.

As used herein, “gas chromatography or “gas chromatographic” includesgas-liquid chromatography and gas-solid chromatography. As used herein,“headspace” includes static headspace techniques and dynamic headspacetechniques. Headspace gas chromatography and gas chromatography areknown to those skilled in the art of analytical chemistry.

As used herein, “samples” includes gas, liquid, and solid materials. Acombinabon of materials may also be used. Examples of samples which maybe used in the method of the invention include, but are not limited to,liquid samples, such as drinking water, beverages, vegetable oils,mineral oils, etc; samples containing a liquid and a solid, such asblood, milk, sewage, polymer dispersions, etc; solid materials whichgive homogenous solutions, such as soluble polymers, inorganic salts,etc; insoluble solid samples, such as high molecular weight olefins,foodstuffs, fruits, tobacco, spices, etc; air, dioxins, PCB's, andpharmaceutical compounds.

Ionic liquids are characterized by a positively charged cation and anegatively charged anion. Generally, any molten salt or mixture ofmolten salts is considered an ionic liquid. Ionic liquids typically haveessentially no vapor pressure, good heat transfer characteristics, arestable over a wide temperature range, and are capable of dissolving awide range of material in high concentrations. As used herein,“essentially no vapor pressure” means that the ionic liquid exhibits avapor pressure of less than about 1 mm/Hg at 25° C., for example lessthan about 0.1 mm/Hg at 25° C.

With respect to the type of ionic liquid, a wide variety ofpossibilities exist. However, the preferred ionic liquids are liquid atrelatively low temperatures, for example, below the melting point of thecompound or sample to be analyzed. For example, the ionic liquid has amelting point of less than 250° C., further example less than 100° C.For example, the ionic liquid has a melting point of less than 30° C. oris a liquid at room temperature.

With regard to viscosity of the ionic liquid, it is important that theviscosity of the ionic liquid is not too high to prevent a homogeneoussolution or dispersion of a compound or sample in an ionic liquid. Forexample, the ionic liquid has a viscosity of less than 500 centipoise(cP), further example, less than 300 cP, or less than 100 cP, asdetermined at 25° C.

In another aspect of the invention the ionic liquid is stable over awide temperature range. Such a ionic liquid may be useful for thetemperature programs used in headspace gas chromatography. The thermalstability of the ionic liquid may be from 150° C. to 400° C., forexample from 200° C. to 300° C.

In a further aspect the invention provides a method according theinvention wherein the ionic liquid has a melting point as describedherein above, e.g. of less than 250° C., a vapor pressure as describedherein above, e.g. less than about 1 mm/Hg at 25° C. and thermalstability as described herein above, e.g. is from 150° C. to 400° C.

The cation present in the ionic liquid can be a single species or aplurality of different species. Both of these embodiments are intendedto be embraced, unless otherwise specified, by the use of the singularexpression “cation.” The cations of the ionic liquid include organic andinorganic cations. Examples of cations include quaternarynitrogen-containing cations, phosphonium cations, and sulfonium cations.

The quaternary nitrogen-containing cations are not particularly limitedand embrace cyclic, aliphatic, and aromatic quaternarynitrogen-containing cations. For example, the quaternarynitrogen-containing cation is an n-alkyl pyridinium, a dialkylimidazolium, or an alkylammonium of the formula R′_(4-X)NH_(X) wherein Xis 0-3 and each R′ is independently an alkyl group having 1 to 18 carbonatoms. It is believed that unsymmetrical cations can provide for lowermelting temperatures. The phosphonium cations are not particularlylimited and embrace cyclic, aliphatic, and aromatic phosphonium cations.For example, the phosphonium cations include those of the formulaR″_(4-X)PH_(X) wherein X is 0-3, and each R″ is an alkyl or aryl groupsuch as an alkyl group having 1 to 18 carbon atoms or a phenyl group.The sulfonium cations are not particularly limited and embrace cyclic,aliphatic, and aromatic sulfonium cations. For example, the sulfoniumcations include those of the formula R′″_(3-X)SH_(X) wherein X is 0-2and each R′″ is an alkyl or aryl group such as an alkyl group having 1to 18 carbon atoms or a phenyl group. The following cations may be used:1-hexylpyridinium, ammonium, imidazolium, 1-ethyl-3-methylimidazolium,1-butyl-3-methylimidazolium, phosphonium, and N-butylpyridinium.

The anion used in the ionic liquid is not particularly limited andincludes organic and inorganic anions. Generally the anion is derivedfrom an acid, especially a Lewis acid. The anions are typically metalhalides as described in more detail below, boron or phosphorusfluorides, alkylsulfonates including fluorinated alkyl sulfonates suchas nonafluorobutanesulfonate, and carboxylic acid anions such astrifluoroacetate and heptafluorobutanoate. The anion is for example Cl⁻,Br⁻, NO₂ ⁻, NO₃ ⁻, AlCl₄ ⁻, BF₄ ⁻, PF₆ ⁻, CF₃COO⁻, CF₃SO₃ ⁻,(CF₃SO₂)₂N⁻, OAc⁻, CuCl₃ ⁻, GaBr₄ ⁻, GaCl₄ ⁻, and SbF₆ ⁻.

Examples of ionic liquids include, but are not limited to, imidazoliumsalts, pyridinium salts, ammonium salts, phosphonium salts, andsulphonium salts. For example imidazolium salts have formula (I)

wherein R¹ and R² are independently selected from the group consistingof a C₁-C₁₈ aliphatic group and a C₄-C₁₈ aromatic group; and A⁻ is ananion.

For example ammonium salts have formula (II)

wherein R³, R⁴, R⁵ and R⁶ are independently selected from the groupconsisting of a C₁-C₁₈ aliphatic group and a C₄-C₁₈ aromatic group; andA⁻ is an anion. For example, R³, R⁴, R⁵ and R⁶ are independentlyselected from the group consisting of ethyl, propyl and butyl.

For example phosphonium salts have formula (III)

wherein R⁷, R⁸, R⁹, and R¹⁰ are independently selected from the groupconsisting of a C₁-C₁₈ aliphatic group and a C₄-C₁₈ aromatic group; andA⁻ is an anion. For example, R⁷, R⁸, R⁹, and R¹⁰ are independentlyselected from the group consisting of ethyl and butyl.

For example pyridinium salts have formula (IV)

wherein R¹¹ is selected from the group consisting of a C₁-C₁₈ aliphaticgroup and a C₄-C₁₈ aromatic group; and A⁻ is an anion. For example R¹¹is ethyl or butyl.

Specific examples of ionic liquids include, but are not limited to,1-butyl-3-methylimidazolium hexafluorophosphate,1-hexyl-3-methylimidazolium hexafluorophosphate,1-octyl-3-methylimidazolium hexafluorophosphate,1-decyl-3-methylimidazolium hexafluorophosphate,1-dodecyl-3-methylimidazolium hexafluorophosphate,1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl)amide,1-hexyl-3-methylimidazolium bis(trifluoromethylsulphonyl)amide,1-hexylpyridinium tetrafluoroborate, 1-octylpyridiniumtetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate,1-methy-3-ethyl imidazolium chloride, 1-ethyl-3-butyl imidazoliumchloride, 1-methy-3-butyl imidazolium chloride, 1-methy-3-butylimidazolium bromide, 1-methy-3-propyl imidazolium chloride,1-methy-3-hexyl imidazolium chloride, 1-methy-3-octyl imidazoliumchloride, 1-methy-3-decyl imidazolium chloride, 1-methy-3-dodecylimidazolium chloride, 1-methy-3-hexadecyl imidazolium chloride,1-methy-3-octadecyl imidazolium chloride, 1-methy-3-octadecylimidazolium chloride, ethyl pyridinium bromide, ethyl pyridiniumchloride, ethylene pyridinium dibromide, ethylene pyridinium dichloride,butyl pyridinium chloride, and benzyl pyridinium bromide.

For example, ionic liquids are 1-octyl-3-methyl-imidazoliumhexafluorophosphate, 1-hexyl-3-methy-imidazolium hexafluorophosphate,1-butyl-3-methyl-imidazolium hexafluorophosphate,1-butyl-3-methyl-imidazolium tetrafluoroborate,1-butyl-3-methyl-imidazolium trifluoromethanesulfonate,1-ethyl-3-methyl-imidazolium trifluoromethanesulfonate, and1-ethyl-3-methyl-imidazolium bis-(trifluoromethanesulfonyl)-amide. Inone aspect of the invention, the ionic liquid is1-octyl-3-methyl-imidazolium hexafluorophosphate or1-hexyl-3-methy-imidazolium hexafluorophosphate.

A mixture of ionic liquids, including binary ionic liquids, may also beused. The ionic liquids may be prepared by any of the methods describedin the art.

The sample or compound to be analyzed by headspace gas chromatography isdissolved or dispersed, for example dissolved, in the ionic liquid. Theamount of ionic liquid is not particularly limited. For example, about 1to about 100 mg of a sample or compound to be analyzed is dissolved ordispersed in about 0.1 to about 5 ml of ionic liquid.

The advantages of the method of the present invention include that: (i)ionic liquids have essentially no vapor pressure, thus, no interferingsolvent peaks are generated by the ionic liquids; (ii) less overpressureis generated inside a sample vial containing an ionic liquid as comparedto a conventional solvent, which reduces seal failure and leakage of thevial; (iii) ionic liquids have high thermal stability which allows theapplication range of headspace gas chromatograph to be expanded; (iv)the high thermal stability of ionic liquids allows the detection limitof headspace gas chromatograph to be expanded; and (v) headspace gaschromatography allows for detection of volatile impurities in ionicliquids; and (vi) ionic liquids may bed analyzed by headspace gaschromatography.

The following non-limiting examples illustrate further aspects of themethod of the invention.

EXAMPLES

Headspace Gas Chromatography Analysis of Volatile Solvents.

Example 1

Four vials are prepared which each contain 39.5 μg ethanol, 45 μg ethylacetate, 39.0 μg cyclohexane, and 43.5 μg toluene, which are dissolvedin 0.1 ml of either one of the following ionic liquids:1-ethyl-3-methyl-imidazolium trifluoromethanesulfonate or1-ethyl-3-methyl-imidazolium bis-(trifluoromethanesulfonyl)-imidate, orone of the following conventional solvents: dimethylsulfoxide orN,N-dimethylacetamide.

The chromatograms are obtained using Agilent Equipment (HeadspaceAutosampler 7694, a Gas chromatograph 5890 equipped with injector (splitmodus) at 150° C., FID at 280° C. and a DB-624 column with 0.53 mm I.D.and 3 μm thickness of stationary phase). The oven temperature of theheadspace sampler is maintained at 100° C. and the vials areequilibrated for 20 min at 120° C. prior to analyzing the gas phase. Thevial pressure is 80 kPa and the flow rate of the mobile phase (helium)is 20 ml/min. GC-temperature program is from 40° C. (1 min), raisingwith 10° C./min to 240° C. and held for 3 minutes at 240° C. Carrier gasis helium at a pressure of 25 kPa.

The chromatogram of each sample is shown in the overlay plot of FIG. 1.The results in FIG. 1 clearly show that ionic liquids may be used inplace of conventional solvents in headspace gas chromatography.Advantageously the ionic liquids due to their high temperature stabilityallow for an expanded application range that includes the actualconventional solvent peaks.

Example 2

Detection Limit for Solvents (e.g. tetrahydrofuran, THF) in DrugSubstance

Two vials are prepared which each contain appr. 100 mg of drugsubstance, which is dissolved in 1 ml 1-butyl-3-methyl-imidazoliummethane sulfonate or in the conventional solvent dimethylsulfoxide(DMSO).

The chromatograms are obtained using Agilent Equipment (HeadspaceAutosampler 7694, a Gas chromatograph 5890 equipped with injector (splitmodus) at 150° C., FID at 280° C. and a DB-624 column with 0.53 mm I.D.and 3 μm thickness of stationary phase). The oven temperature of theheadspace sampler is maintained at 100° C. and the vials areequilibrated for 30 min prior to analyzing the gas phase. The vialpressure is 80 kPa and the flow rate of the mobile phase (helium) is 20ml/min. GC-temperature program is from 40° C. (2 min isothermal),raising with 5° C./min until 125° C., following with 30° C./min to 240°C. and held for 3 minutes at 240° C. isothermal. Carrier gas is heliumat a pressure of 32 kPa.

The chromatogram of each sample is shown in the overlay plot of FIG. 2.The results in FIG. 2 clearly show the advantage of using an ionicliquid as a solvent. The detection limit for THF in the drug substanceis significantly improved (chromatogram 1).

Example 3

Quantification of High Boiling Compounds

A vial is prepared which contains appr. 23.8 μg dimethyl formamide(DMF), 151.8 μg dimethylsulfoxide (DMSO), 33.2 μg dimethylacetamide(DMA), and 3.9 μg diethyleneglycoldimethylether (DIGLYME), which isdissolved in 1 ml of EMIM bis-(trifluoromethanesulfonyl)-imidate (II). Asecond vial contains only 1 ml of EMIMbis-(trifluoromethanesulfonyl)-imidate for comparison (I).

The chromatograms are obtained using Agilent Equipment (HeadspaceAutosampler 7694, a Gas chromatograph 5890 equipped with injector (splitmodus) at 210° C., FID at 280° C. and a DB-624 column with 0.53 mm I.D.and 3 μm thickness of stationary phase). The oven temperature of theheadspace sampler is maintained at 200° C. and the vials areequilibrated for 20 min prior to analyzing the gas phase. The vialpressure is 100 kPa and the flow rate of the mobile phase (helium) is 20ml/min. GC-temperature program is from 40° C. (2 min isothermal),raising with 8° C./min until 180° C., followed with 15° C./min to 240°C. and hold for 3 minutes at 240° C. isothermal. Carrier gas is heliumat a pressure of 32 kPa.

The chromatogram of each sample is shown in the overlay plot of FIG. 3.The results in FIG. 3 (retention times: appr. 10.5 min for DMF, 12.5 minfor DMSO, 13.0 for DMA and 13.5 for DIGLYME). Advantageously highboiling solvents can easily be detected while using an ionic liquid assolvent.

Example 4

Detection of Impurities Co-eluting with Solvents “Hidden Compounds”

Two vials are prepared which each contained appr. 10 mg of drugsubstance, which is dissolved in 100 μl EMIMbis-(trifluoromethylsulfonyl)-imidate (IL1) or in the conventionalsolvent dimethylacetamide (DMA).

The chromatograms are obtained using Agilent Equipment (HeadspaceAutosampler 7694, a Gas chromatograph 5890 equipped with injector (splitmodus) at 150° C., FID at 280° C. and a DB-624 column with 0.53 mm I.D.and 3 μm thickness of stationary phase). The oven temperature of theheadspace sampler is maintained at 150° C. and the vials areequilibrated for 30 min prior to analyzing the gas phase. The vialpressure is 80 kPa and the flow rate of the mobile phase (helium) is 20ml/min. GC-temperature program is from 40° C. (2 min isothermal),raising with 5° C./min until 125° C., followed with 30° C./min to 240°C. and held for 3 minutes at 240° C. isothermal. Carrier gas is heliumat a pressure of 32 kPa.

The chromatogram of each sample is shown in the overlay plot of FIG. 4.The results in FIG. 4. clearly show that “hidden compounds” (e.g. peakX) that usually co-elute with classical solvents can now be detected andquantified while using an ionic liquid as new solvent (chromatogram 1).

While the invention has been described with particular reference tocertain embodiments thereof, it will be understood that changes andmodifications may be made by those of ordinary skill within the scopeand spirit of the following claims:

1. A method of performing headspace gas chromatography using ionicliquid as solvents comprising the steps of dissolving or dispersing asample in at least one ionic liquid, wherein the ionic liquid is amolten salt selected from the group consisting of1-butyl-3-methylimidazolium hexafluorophosphate,1-hexyl-3-methylimidazolium hexafluorophosphate,1-octyl-3-methylimidazolium hexafluorophosphate,1-decyl-3-methylimidazolium hexafluorophosphate,1-dodecyl-3-methylimidazolium hexafluorophosphate,1-ethyl-3-methylimidazolium bis((trifluoromethyl)sulphonyl)amide,1-hexyl-3-methylimidazolium bis((trifluoromethyl)sulphonyl)amide,1-hexylpyridinium tetrafluoroborate, 1-octylpyridiniumtetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate,1-methyl-3-ethyl imidazolium chloride, 1-ethyl-3-butyl imidazoliumchloride, 1-methyl-3-butyl imidazolium chloride, 1-methyl-3-butylimidazolium bromide, 1-methyl-3-propyl imidazolium chloride,1-methyl-3-hexyl imidazolium chloride; 1-methyl-3-octyl imidazoliumchloride, 1-methyl-3-decyl imidazolium chloride, 1-methyl-3-dodecylimidazolium chloride, 1-methyl-3-hexadecyl imidazolium chloride,1-methyl-3-octadecyl imidazolium chloride, 1-methyl-3-octadecylimidazolium chloride; ethyl pyridinium bromide, ethyl pyridiniumchloride, ethylene pyridinium dibromide, ethylene pyridinium dichloride,butyl pyridinium chloride, benzyl pyridinium bromide, and mixturesthereof, and volatilizing the volatile components of the sample byheadspace gas chromatography.
 2. The method according to claim 1 whereinthe ionic liquid is selected from the group consisting of1-octyl-3-methyl-imidazolium hexafluorophosphate,1-hexyl-3-methy-imidazolium hexafluorophosphate,1-butyl-3-methyl-imidazolium hexafluorophosphate,1-butyl-3-methyl-imidazolium tetrafluoroborate,1-butyl-3-methyl-imidazolium trifluoromethanesulfonate,1-ethyl-3-methyl-imidazolium trifluoromethanesulfonate, and1-ethyl-3-methyl-imidazolium bis-(trifluoromethanesulfonyl)-amide.
 3. Amethod of performing headspace gas chromatography using ionic liquid assolvents comprising the steps of dissolving or dispersing a sample in atleast one ionic liquid, wherein the ionic liquid is a molten saltselected from the group consisting of an imidazolium salt having formula(I)

wherein R¹ and R² are independently selected from the group consistingof a C₁-C₁₈ aliphatic group and a C₄-C₁₈ aromatic group, and A⁻ is ananion; an ammonium salt having formula (II)

wherein R³, R⁴, R⁵ and R⁶ are independently selected from the groupconsisting of a C₁-C₁₈ aliphatic group and a C₄-C₁₈ aromatic group, andA⁻ is an anion; a phosphonium salt having formula (III)

wherein R⁷, R⁸, R⁹, and R¹⁰ are independently selected from the groupconsisting of a C₁-C₁₈ aliphatic group and a C₄C₁₈ a aromatic group, andA⁻ is an anion; a pyridinium salt having formula (IV)

wherein R¹¹ is selected from the group consisting of a C₁-C₁₈ aliphaticgroup and a C₄-C₁₈ aromatic group, and A⁻ is an anion; and mixturesthereof.